Abstract: The fluorescent properties of 1,3-diphenyl pyrazoline(DPP) and 1-phenyl-3-(4-nitrophenyl) pyrazoline(PNP) were studied at room temperature(293 K) and 77 Krespectively.In non-polar solvents the intensity of the fluorescence of DPP at 77K was much lower than that at room temperature and PNP showed no emission at 77K.These phenomena indicate that it is very important for luminescent properties of the compounds that in excited state the conformation of the molecules relaxes from a planar to a twisted one.The studies of the modified compounds,inhibiting the rotation of the phenyl groups at different positions in DPP and PNP,verified the expression above.From experimental results it can also be concluded that there are different key twisting positions to form luminescent conformation between DPP and PNP.It is very important for forming the luminescent state of DPP to twist the phenyl at N-1 position,in contrast with that,twisted phenyl at C-3 is more important for establishing the luminescent state in PNP.

Key words: Twisted luminescent conformation, 1,3-Diphenyl pyrazoline derivatives, Fluorescence